full Paper - Nguyen Dang Binh

Table 1: Summary of GRAB features
Features Value Units
Worst case Typical
Continuous force 4 6-7 N
Peak force 20 30 N
Stiffness 2 6-8 N/mm
Position resolution 100 50 µm
Workspace 400x400x600 mm
Maximum force error 2-4 %
Coherence (max. error) 0.5/100 mm/mm
force/acceleration effects during motion, manipulation
and exploration of objects.
Position accuracy: the accuracy of the spatial position is
important in order to achieve an highly collocated system.
Objects and distances in a collocated environment should
exactly re¤ect what the interface measure in terms of position.
Erroneous measurements will result in distorted
environments (curved planes, misaligned object, missing
contact during surface exploration).
Posture calibration: multi-haptic environments requires
sophisticated algorithms to detect in an accurate way the
relative position of the force display interfaces. Any error
in this procedure will result in a “drunk like” representation
of the object (i.e. the object position differs according
to the exploring £nger).
4. The haptic device
In order to overcome these issues, a novel kind of device has
been built within the GRAB project. The device kinematics
has been shaped in such a way that the available workspace
is as large as a desktop environment even when two interacting
interfaces are put in the operating environment. The system
can be worn on the £ngertips by means of thimbles. The
device’s shape has been designed in order to limit at a minimum
the interference problem even when the user’s £ngertips
are very close each other. The haptic interface has been
described in detail in [AMA03] and has been patent in 2004.
In Table 1, a summary of the features is given. The interface
is capable of replicating two independent (3D) force vectors
on two £ngertips, with a workspace covering a large part of
the user desktop (400mm x 400mm x 600mm) and with high
force feedback £delity to perceive tiny geometrical details of
the objects. The peculiar cinematic of the devices facilitates
the integration of multiple devices on the same desktop. A
snapshot of the arm device is shown in Fig. 3. Evenifthe
system has been developed with two arm working, several
c○ The Eurographics Association 2005.
Massimo Bergamasco / Haptic Rendering: Control Strategy
61
Figure 3: CAD model of one arm
identical robotic structures may be let operating on the same
table.
Each structure has a total of 6 Degree of Freedoms
(DOF’s). The £rst three DOFs exert the haptic feedback and
could be used to track the position of the £ngertip in the
workspace. These DOFs have been implemented with two
rotational joint and one prismatic joint (RRP). The remaining
3 degrees are completely passive and can be used to track
the £ngertip orientation (degrees of mobility). These degrees
of mobility are realized by rotational joints (RRR) in such a
way their rotation axes coincides in a common point which
is aligned with the axis of the prismatic joint.
5. Integrating the environment
In Fig. 4 is shown the whole GRAB system, composed by
following main components:
two arms, having the same kinematics and symmetrically
placed on the same desktop. The user interface of the
two haptic interface is a couple of thimbles which can be
adapted to any £nger size and/or replaced with speci£c
interactive pens;
a motor driver and supply group including: six miniature
PWM servo ampli£ers for DC brush motors, custom
mixed analog/digital ICs and hybridised power stage, capable
to supply the required motors current (continuous
current of 10 A, peak current of 20 A);
a Control Unit, based on a x86 standard, Single Board
Computer (SBC) which provides resource for running a
Real - Time operating system required by Low Level Controller;
Haptic Geometric Modeller, which receives the £ngers
position from Control Unit and provides to sends back the
forces for simulating virtual environment.
6. System calibration procedures
System calibration procedures are implemented in order to
maximize the precision in tracking £nger position. It consists
of two different calibration procedures.